A method using optical interference is widely known as a method for measuring the amount of displacement or moving distance of an object under measurement (Meas. Sci. Technol., 9 (1998), 1024-1030). An example of the method is shown in FIG. 12. With an interferometer shown in FIG. 12, two-frequency orthogonally polarized beams 302, of which polarizations are orthogonal each other and the difference of the optical frequency between the two beams is 20 MHz, is emitted from a laser head 301. The polarized beam is split into two polarization components by a polarizing beam splitter 303. An S-polarized beam 304 reflects off a polarization beam splitter 303 and then a rectangular prism 305, then enters the polarizing beam splitter 303 as a reference beam. A P-polarized beam 306 passes through the polarizing beam splitter 303, reflects off a rectangular prism 307 arranged on an object under measurement 400, and then enters the polarizing beam splitter 303. Both reflected beams are combined by the polarizing beam splitter 303. The combined beam passes through a polarizing plate 308 having a polarization angle of 45 degrees with respect to the polarization directions of both reflected beams, and then is subjected to heterodyne interference. The heterodyne interference light is received by a photoelectric transducer 309 and then converted to an electrical signal 310. A frequency fM of the heterodyne interference signal 310 is given by Formula (1) with the addition of the Doppler shift frequency in relation to a moving velocity V of the object under measurement 400.fM=fB±NV/μ  (1)where fB=20 MHz and λ is a wavelength of the laser beam. Further, N (=2, 4, . . . ) is a constant determined by the number of round-trip propagations through the optical path. In FIG. 12, N=2. On the other hand, a beat signal 311 having a frequency (fB) of 20 MHz is outputted from the laser head 301 as a reference signal. A measured heterodyne interference signal 310 and the reference signal 311 are inputted to a phase detector 312. A moving velocity V and a moving distance 400d of the object under measurement 400 are obtained from the phase difference between both signals and outputted as a moving distance output signal 313.    Nonpatent Reference 1: Meas. Sci. Technol., 9 (1998), 1024-1030